1. Field of the Invention
This invention relates to picture image forming apparatus utilized for a heat transfer printing type copying machine or the like.
2. Description of the Prior Art
Recently, a heat transfer printing type color copying machine has been developed in which color copying is made by using heat transfer ink ribbons of a plurality of colors on an ink donor sheet. Briefly stated, according to this type of the copying machine, a manuscript is scanned with optical scanning means for reading the picture image data of the manuscript as light color signals, the read out picture image data are converted into color data corresponding to respective types of the transfer printing ink ribbons, and the color data of respective inks are temporarily stored in memory means. The color data thus stored are sequentially read out, inking units corresponding to the heat transfer printing ink ribbons are used in accordance with read out color data and the inks are heat transfer printed with a heat sensitive head, thus effecting color copying by sequentially transfer printing, one color after another, onto a sheet of paper hereinafter called copying paper.
In the copying machine of this type, during the transfer printing the copying paper is conveyed while being clamped between a platen and the ink ribbon. After transfer printing, the copying paper is conveyed between a push roller and the platen. However, since the ink ribbon is moved under a constant tension, when the tension is adjusted to be suitable for thin paper, if thick or stiff paper were used, the leading end of the paper would collide against a ribbon guide with the result that the paper clogs the ribbon guide, or the paper is entrapped or misalignment of transfer printed copies would occur. Where the tension is made high at the start of the machine, at the time of transfer printing of a thin paper, adequate tension cannot be obtained causing misalignment of transfer printed picture images.
Both ends of the ink ribbon are wound on a pair of cores independently driven by drive means and the ink ribbon is run between the cores under tension and stopped. On the ink ribbon are coated color agents comprising a combination of yellow, magenta, cyan, and black or a combination of yellow, magenta and cyan. During the transfer printing operation, after completing the transfer printing of a color agent of a given color, the copying paper is run in the opposite direction to an original position and at the same time the ink ribbon is run to a position at which the transfer printing of the next color agent is to be started.
In this case, after detecting, for example, bar codes formed on the ink ribbon, the tensions applied to the ink ribbon in the opposite directions created by both drive means are made equal to stop the ink ribbon. However, due to the inertia of the drive means and the winding cores, the ribbon tends to over run so that it has been difficult to instantaneously stop the paper at a desired position. Consequently, the color agents must be coated over a range wider than a range (a length in the running direction) of the color agent contributing directly to the formation of the picture image whereby the ink ribbon is not used efficiently. Otherwise, it becomes impossible to superpose color agents of a plurality of colors by successive transfer printing operations.
Furthermore, with the prior art heat transfer printing apparatus, the ink ribbon often slacks temporarily during operation which causes degradation of the picture quality, such as blurring of the transfer printed picture images. Especially, in the multicolor heat transfer system since there are such motions as reciprocation of the recording head and forward and reverse runnings of the ink ribbon, the temporal slack of the ribbon greatly decreases the picture quality.